The invention relates generally to an improved method for retreading the running surface of a vehicle tire, and more particularly to a novel method for producing a rubber and cloth tread mold and utilizing that tread mold around a buffed tire carcass covered with uncured rubber to accomplish retreading.
Heretofore, various methods have been employed for retreading vehicle tires. Commonly, worn tires are recapped by the warm vulcanization method, wherein an uncured tread cover or "camel-back" is applied to the buffed crown surface of the previously cured tire casing or carcass and the assembly is vulcanized at an elevated temperature in a suitable mold. A major disadvantage of this type of retread is that the fixed mold requires stretching and expansion of the tire during retreading because the tire must be smaller than the mold to insert it in the mold. Steel radial tires do not stretch; thus, mold size is critical, and numerous molds are necessary to accommodate the various steel radial tire dimensions which vary not only from one specified tire size to another but among tires of the same nominal size made by different manufacturers. Any deformation or distortion of the tire casing during the curing operation will tend subsequently to rupture or materially weaken the vulcanized bond between the cured "camel-back" and the crown surface of the tire after it is removed from the vulcanizing mold. In addition, the elevated vulcanizing temperature tends to weaken and deteriorate the previously cured tire casing.
The prior art has sought to overcome shortcomings in the warm vulcanization recapping method by various "cold" recapping methods utilizing both precured rubber treads and uncured rubber. One approach to cold recapping is disclosed in my U.S. Pat. No. 4,269,644, which involves a cold tire recapping method utilizing a precured tread strip and a rubber bonding composition between the strip and the prepared tire carcass.
There are disadvantages associated with utilization of such previously manufactured precured rubber tread material, including the cost of that material as compared to the cost of uncured rubber, the need to maintain a large inventory of widths and tread designs, the difficulty of producing a tire having good physical appearance at the transition of tread and sidewall and the difficulty of retreading traction tires with deep sidewall voids.
A different recapping approach is disclosed in the U.S. Pat. No. 4,185,056 to Detwiler, which discloses the formation of a rubber mold and utilization of that mold together with uncured rubber to retread the prepared tire carcass. The patent to Detwiler discloses formation of the tread mold by utilizing a new mold tire which is coated with a mold release and then covered with a very loosely woven "fabric" network of strands, the transverse strands of which are intended to serve as air venting conduits during the curing operation. Such transverse strands become embedded in the cured female mold and thereafter serve little or no function during recapping utilizing the mold but weaken the tread-forming portion of the rubber mold.
Failure to provide a structure and/or method which effectively purges air and gases at the interface between the tread mold and the new tread mold during recapping causes excessive porosity within the new tread, which creates accelerated wear and tread to tire carcass bonding problems. Furthermore, utilization of transverse strands which become embedded in the mold results in limited mold life because the exposed rubber mold surface does not wear well, and tread groove-forming protrusions in the female retread mold tend to break off during use.
Another prior art approach similar to the Detwiler patent is disclosed in the U.S. Pat. Nos. 3,983,193 and 4,053,265 to Wulker. The Wulker patents disclose a resilient mold which has an unstretched inner diameter smaller than the outer diameter of the cured tread on the recapped tire. In use, the Wulker mold is stretched in order to place it around the tire carcase and uncured rubber assembly and the mold contracts to normal size to impress its pattern on the tread rubber. The Wulker patents disclose provision for purging air and other gases from the mold-tread interface comprising holes passing radially through the mold.
Additional limitations inherent in the prior art rubber molds and methods for recapping with uncured rubber typified by the patents to Detwiler U.S. Pat. No. 4,185,056 and Wulker U.S. Pat. Nos. 3,983,193 and 4,053,265 include problems in fully seating the mold during recapping, difficulties in mounting the mold on a tire carcass and in removing the mold after curing of the recap tread is complete and the need for numerous molds having slightly different diameters in order to accommodate variations among tires being recapped.
Seating problems occur because of sealing contact between the carcass sidewalls and the sidewall-forming portion of the rubber mold which will not easily slide along the carcass sidewall and thereby permit the entire tread-forming portion of the mold to seat in the uncured rubber as the recap tread is formed. A typical prior art approach to this problem involves repeated pressurization and evacuation of the envelope enclosing the carcass and mold during the curing cycle in an effort to "work" the mold into its fully seated position. This practice is inconsistent, however, with the need to maintain constant pressure during the early stage of recap curing in order to achieve optimum curing results.
Problems in mounting the female mold on the uncured rubber and tire carcass and in removal of the female mold from the cured recapped tire reduce production efficiency and removal problems frequently result in damage to the newly recapped tire and/or the mold.